A nanofiltration membrane and a reverse osmosis membrane have such a filmy structure that a pore size of membrane thereof is in an order of from nanometers to angstroms or that being considered to have no clear pore. Therefore, their resistance to filtration is large and permeation flow rate of water is apt to become small. Accordingly, as to a nanofiltration membrane and a reverse osmosis membrane, a structure of a composite separation membrane having both of high water permeation property and separation property has been preferably used, wherein thin film having a separation function is formed as thin as possible and without deficiency on the surface of a porous support membrane being excellent in mechanical strength and water permeation property. In addition, a polymer which constitutes the separation layer has been demanded to be excellent in the resistance to chemicals or, particularly, in the resistance to chlorine and alkali in view of washability and stability for a long-term use.
As to the structure of the conventional main composite separation membranes, there is a structure wherein thin membrane of cross-linked aromatic polyamide is formed on the surface of a porous support membrane by means of an interfacial polymerization method. For example, in Patent Document 1, there is disclosed a composite product in a sheet form wherein thin film of cross-linked polyamide is formed on the surface of a porous support membrane by means of interfacial polymerization.
In Patent Document 2, there is disclosed a hollow fiber composite separation membrane wherein thin film of cross-linked polyamide is formed on the surface of a porous support membrane in a hollow fiber form by means of interfacial polymerization.
In Patent Document 3, there is also disclosed an art for forming a hollow fiber composite separation membrane wherein thin film of cross-linked polyamide is formed on the surface of a porous support membrane in a hollow fiber form by means of interfacial polymerization. In said art, a step of impregnating a liquid containing a fluorine compound is added to a step of compositing by means of interfacial polymerization so as to form a hollow fiber composite separation membrane having more uniform separation layer.
As to a synthetic polymer other than the polyamide-type materials which can be applied to a nanofiltration membrane or a reverse osmosis membrane, there is a polymer having an ionic functional group such as sulfonic group in a molecule. For example, in Patent Document 4, there is disclosed an art for preparing a composite separation membrane wherein a sulfonated polyarylene ether is dissolved in a solvent consisting of formic acid and the resulting coating solution is applied on the surface of a porous support membrane followed by drying to form a coat.
However, although the nanofiltration membrane and reverse osmosis membrane using a polyamide-type composite separation membrane as mentioned in Patent Document 1 are excellent in their salt rejection property and water permeation property, their resistance to chlorine is low whereby it is impossible to treat water containing sodium hypochlorite and it is also impossible to be washed with chlorine. Therefore, it is necessary to subject a supplying solution from which sodium hypochlorite has been once removed to a desalting treatment using the separation membrane and then to add sodium hypochlorite again to the resulting filtered solution, which leads to a problem that a filtering process is complicated and the cost therefor is high.
In Patent Documents 2 and 3, there is also a disadvantage that resistance to chlorine is low because of a polyamide-type composite separation membrane. Moreover, there is also a problem that a process wherein the structure formation is conducted by an interfacial polymerization reaction in a step of manufacturing a composite separation membrane of a hollow fiber type is complicated a compared with a flat membrane or a sheet-shaped product.
A composite separation membrane having a sulfonated polyarylene ether (SPAE) in a separation layer as in Patent Document 4 is very excellent in the resistance to chlorine due to its high chemical stability of the polyarylene ether molecule skeleton. Therefore, it is preferred in practical use because it can be washed with sodium hypochlorite.
However, as pointed out in Non-Patent Document 1 for example, since SPAE is similar to polysulfone or polyether sulfone which is a polymer material for common porous support membranes in terms of the chemical structure, most of solvents which can dissolve SPAE also can dissolve polysulfone or polyether sulfone. When the solvent as such is used as a coating solution and applied on a porous support membrane, there is resulted a problem that the porous support membrane is dissolved or significantly swollen whereby no composite membrane is prepared.
Accordingly, it is inevitable to select a limitative solvent (lower carboxylic acid such as formic acid, alcohol, alkylene diol or triol, or alkylene glycol alkyl ether) which does not invade a porous support membrane formed of polysulfone or polyether sulfone. However, such a solvent should also tends to become low solubility to SPAE. Particularly, the allowable range of solubility of a solvent to SPAE having more rigid molecular skeleton is narrow. When a composite separation membrane is prepared using such a solvent having insufficient solubility, a coat of SPAE cannot firmly adheres to a porous support membrane and the separation property tends to become insufficient whereby there is a problem that lowering of the property for long period due to exfoliation is apt to happen.